USDA Chief Veterinary Officer John Clifford said Tuesday afternoon that the cow did not enter the human food chain and that all U.S. meat and dairy supplies are safe. Further mitigating the risk to the public, milk does not transmit BSE.

According to the USDA, the animal's carcass is being held under state authority at a California rendering facility and will be destroyed. "It was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health," Clifford said.

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About 40,000 cattle were tested in the 12 months ended Sept. 30, down from 399,575 in 2005, according to data from the USDA. The agency said the drop was related to a temporary surge in testing done for research.

The decline in testing means the U.S. is relying more on other prevention methods, such as restrictions on what can be fed to cattle, and that is worrisome because those other safeguards are not foolproof, Chris Waldrop, director of the Food Policy Institute of the Consumer Federation of America, a Washington-based safety advocacy group, said in an interview.

Major markets for U.S. beef from Canada to Japan stayed open to imports on Wednesday after the first U.S. discovery of mad cow disease in six years on assurances that rigorous surveillance had safeguarded the food system.

South Korea’s No. 2 and No. 3 supermarket chains, Home Plus and Lotte Mart, said they halted sales of U.S. beef to calm worries among South Koreans. But within hours, Home Plus had resumed sales and cited a government announcement of increased inspections. Lotte kept its suspension in place.

“We stopped sales from today,” said Chung Won-hun, a Lotte Mart spokesman. “Not that there were any quality issues in the meat but because consumers were worried.”

South Korea is the world’s fourth-largest importer of U.S. beef, buying 107,000 tons of the meat worth 3 million in 2011.

The new case of mad cow disease is the first in the U.S. since 2006. It was discovered in a dairy cow in California, but health authorities said Tuesday the animal was never a threat to the nation’s food supply.

The AP confirms that the discovery of mad cow disease in a dairy cow carcass at a California rendering facility was purely a matter of luck:

Tests are performed on only a small portion of dead animals brought to the transfer facility near Hanford.

The cow had died at one of the region's hundreds of dairies, but hadn't exhibited outward symptoms of the disease: unsteadiness, incoordination, a drastic change in behavior or low milk production, officials said. But when the animal arrived at the facility with a truckload of other dead cows on April 18, its 30-month-plus age and fresh corpse made her eligible for USDA testing.

"We randomly pick a number of samples throughout the year, and this just happened to be one that we randomly sampled," Baker Commodities executive vice president Dennis Luckey said. "It showed no signs" of disease.

The samples went to the food safety lab at the University of California, Davis on April 18. By April 19, markers indicated the cow could have bovine spongiform encephalopathy (BSE), a disease that is fatal to cows and can cause a deadly human brain disease in people who eat tainted meat. It was sent to the USDA lab in Iowa for further testing.

On Tuesday, federal agriculture officials announced the findings: the animal had atypical BSE. That means it didn't get the disease from eating infected cattle feed, said John Clifford, the Agriculture Department's chief veterinary officer.

It was "just a random mutation that can happen every once in a great while in an animal," said Bruce Akey, director of the New York State Veterinary Diagnostic Laboratory at Cornell University. "Random mutations go on in nature all the time."

In humans, experts say it can occur in one in 1 million people, causing sponge-like holes in the brain. But they say not enough is known about how and how often the disease strikes cattle.

The disease cannot be transmitted by contact among cows, and experts say it's unclear whether this rare type of BSE ever has been transmitted from a cow to a human by eating meat.

In an interview on CNN, Agriculture secretary Tom Vilsack proclaimed, "I am going home and I am having beef for dinner, and that is no lie." In a statement released earlier, the secretary said, "The systems and safeguards in place to protect animal and human health worked as planned to identify this case quickly, and will ensure that it presents no risk to the food supply or to human health."

The chairman of the Senate Finance Committee, Sen. Max Baucus of Montana, was less assured, Bloomberg reported: "Where'd this cow come from? What's its feed?" he asked.

The Canadian Food Inspection Agency said Tuesday both countries have implemented science-based measures to protect animal and human health. The agency also noted that U.S. officials have confirmed that no part of this animal's carcass entered the food system.

The United States closed its border to Canadian beef in 2003 after sick cows were detected in Canada.

Baker Commodities processes animal byproducts, including dead cattle, to make commercial commodities. According to the company's website, these products include "high-protein ingredients for poultry feed and pet food, and tallow, a valuable ingredient in soaps, paints, cosmetics, and more." While a diseased animal could have been processed without being tested, it's unclear if an infected cow would have been rendered for feed or pet food at the facility.

The Associated Press adds:

Michael Marsh, chief executive of Western United Dairymen, said it was an adult cow over 30 months old, not a downed or sick animal, and it appeared normal when it was last observed. He said the cow was first tested on April 18.

Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, told ABC News that it was unlikely any more cows would contract BSE. “Mad cow occurs in animals as it does in humans — rarely and sporadically. At this point, I would not expect there to be another cow to be found,” he said.

The animal tested positive for a case of "atypical" BSE, "a very rare form of the disease not generally associated with an animal consuming infected feed," the USDA said.

The Associated Press reports:

"There is really no cause for alarm here with regard to this animal," Clifford told reporters at a hastily convened press conference.

Clifford did not say when the disease was discovered or exactly where the cow was raised. He said the cow was at a rendering plant in Central California when the case was discovered through regular USDA sample testing.

Mad cow disease, or bovine spongiform encephalopathy (BSE), can be fatal to humans who eat tainted beef. The World Health Organization has said that tests show that humans cannot be infected by drinking milk from BSE-infected animals.

The disease is always fatal in cattle, however. There have been three confirmed cases of BSE in the United states, in a Canadian-born cow in 2003 in Washington state, in 2005 in Texas and in 2006 in Alabama.

In people, eating meat contaminated with BSE is linked to variant Creutzfeldt-Jakob Disease, a rare and deadly nerve disease. A massive outbreak of mad cow disease in the United Kingdom that peaked in 1993 was blamed for the deaths of 180,000 cattle and more than 150 people.

There have been a handful of cases of variant Creutzfeldt-Jakob Disease confirmed in people living in the United States, but those were linked to meat products in the United Kingdom and Saudi Arabia, according to the Centers for Disease Control and Prevention.

The USDA's BSE surveillance program tests approximately 40,000 high-risk cattle each year, which only allows detection of BSE at a level of one case for every one million adult cattle. However, it exceeds the World Organization for Animal Health's (OIE) recommended testing levels assigned to the U.S.

Analysts said the biggest risk to the beef market from a confirmed case would be if large international beef customers such as Japan and South Korea were to impose a temporary ban, but they said that new bans were unlikely if the animal was kept out of the food chain.

CNN's Elizabeth Cohen reports that it takes about 15 years for a human to show signs of mad cow disease. Therefore, once a person exhibits signs of the disease, it is way too late to take the meat out of the food system. "If even one person ate U.S. meat and got sick from mad cow disease, it would just be devastating," she says.

The National Cattlemen’s Beef Association released a statement Tuesday afternoon, concluding, "The bottom line remains the same – all U.S. beef is safe." "The U.S. beef community has collaborated with and worked with animal health experts and government to put in place multiple interlocking safeguards over the past two decades to prevent BSE from taking hold in the United States," the organization said.

Flip through the slideshow below for the history of mad cow incidences in the U.S. since the first case in December 2003:

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History Of Mad Cow Incidences In The U.S.

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The first confirmed case of mad cow disease in the U.S. involved an animal from a farm in Mabton, Wash. The Holstein had been imported in 2001 from Alberta, Canada, along with 70 other cows. The animal was a "downer," which means it was unable to walk when it reached the slaughterhouse, a condition that mandates automatic testing from the USDA. Following the determination of mad cow, the processor, Vern's Moses Lake Meats, voluntarily recalled 10,410 pounds of raw beef amid concerns that products might be tainted. These cows, pictured at Sunny Dene Ranch in Mabton, were quarantined in the following months during an investigation.

The second confirmed occurrence of the disease in the U.S. was linked to a farm in Texas, though it appears to be the country's first home-grown case. The New York Times reported that the animal was about 12 years old at the time of its death. It had spent the entirety of its life on the same Lone State ranch until it was taken to pet food plant Champion Pet Food, Inc. in Waco, Texas, where it died in November of 2004. The animal was a "downer" which pet food outfits often take since the USDA prohibits such cows for human consumption. Testing for disease is still mandatory, and meat from the animal did not enter the food supply, thanks to safeguards.
Photo by Flickr user MdenHoedt.

A cow in Alabama was the third confirmed case of mad cow in the U.S. CBS reported that USDA head veterinarian John Clifford assured the public that meat from the animal had not entered the food supply for people or animals. The animal was also a "downer," which led to it being euthanized and tested.
Photo by Flickr user Shan213.

Earlier this year, two reported cases of mad cow-related illness in Marin County, Calif., one fatal, turned out not to be linked to the disease. The scare, however, leaves Americans shaken.

The fourth confirmed case of the disease was traced to a dairy cow in central California.

"As part of our targeted surveillance system, the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection Service (APHIS) has confirmed the nation's fourth case of bovine spongiform encephalopathy (BSE) in a dairy cow from central California. The carcass of the animal is being held under State authority at a rendering facility in California and will be destroyed. It was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health. Additionally, milk does not transmit BSE.

"The United States has had longstanding interlocking safeguards to protect human and animal health against BSE. For public health, these measures include the USDA ban on specified risk materials, or SRMs, from the food supply. SRMs are parts of the animal that are most likely to contain the BSE agent if it is present in an animal. USDA also bans all nonambulatory (sometimes called "downer") cattle from entering the human food chain. For animal health, the Food and Drug Administration (FDA) ban on ruminant material in cattle feed prevents the spread of the disease in the cattle herd.

"Evidence shows that our systems and safeguards to prevent BSE are working, as are similar actions taken by countries around the world. In 2011, there were only 29 worldwide cases of BSE, a dramatic decline and 99% reduction since the peak in 1992 of 37,311 cases. This is directly attributable to the impact and effectiveness of feed bans as a primary control measure for the disease.

"Samples from the animal in question were tested at USDA's National Veterinary Services Laboratories in Ames, Iowa. Confirmatory results using immunohistochemistry and western blot tests confirmed the animal was positive for atypical BSE, a very rare form of the disease not generally associated with an animal consuming infected feed.

"We are sharing our laboratory results with international animal health reference laboratories in Canada and England, which have official World Animal Health (OIE) reference labs. These labs have extensive experience diagnosing atypical BSE and will review our confirmation of this form of the disease. In addition, we will be conducting a comprehensive epidemiological investigation in conjunction with California animal and public health officials and the FDA.

"BSE is a progressive neurological disease among cattle that is always fatal. It belongs to a family of diseases known as transmissible spongiform encephalopathies. Affected animals may display nervousness or aggression, abnormal posture, difficulty in coordination and rising, decreased milk production, or loss of body weight despite continued appetite.

"This detection in no way affects the United States' BSE status as determined by the OIE. The United States has in place all of the elements of a system that OIE has determined ensures that beef and beef products are safe for human consumption: a mammalian feed ban, removal of specified risk materials, and vigorous surveillance. Consequently, this detection should not affect U.S. trade.

"USDA remains confident in the health of the national herd and the safety of beef and dairy products. As the epidemiological investigation progresses, USDA will continue to communicate findings in a timely and transparent manner."